Bright Planets and Random Meteors

Bright Planets & Random Meteors

This week's new Moon sets a dark stage for a sporadic
meteor show featuring a cast of eye-catching stars and planets.

September
28, 2000 -- It can be hard to wake up on dark autumn mornings,
especially if what lies ahead is a long commute to work or school.
But a slow-rising Sun isn't all bad. This week it offers drowsy
commuters something to look forward to -- a breakfast buffet
of morning stars, planets, and meteors.

At 5:30 a.m. local time the planets Jupiter and Saturn, the
red star Aldebaran, and the delicate Pleiades form an eye-catching
rectangle nearly overhead at mid-northern latitudes. It's enough
to make hurried travelers forget their cell phones! Southbound
motorists are likely to see the constellation Orion astride the
southeastern horizon not far from low-hanging Sirius, the brightest
star in the sky. Sirius is so bright that it's often mistaken
for an airplane, but watch carefully. If it doesn't move it is
probably the Dog Star.

Right: This fanciful graphic by Duane
Hilton shows the pre-dawn sky as it might appear this week
to a southbound freeway commuter. Click
on the image to view a larger version with objects in the sky
labeled.

The bright planets and stars make it a good time for freeway
stargazing -- at least for passengers. Swerving drivers are better
off with their eyes on the road.

It's also a rewarding week for early-rising meteor watchers.
Wednesday's New Moon sets the stage for the annual maximum of
sporadic meteors in the northern hemisphere. The absence of moonlight
won't make much difference to drivers on city-lit freeways, but
rural observers can expect to see 5 to 15 meteors per hour. A
few bright shooting stars, like the spectacular fireball many
people saw in Southern California on Wednesday, will likely dazzle
urbanites as well.

Sporadic meteors are caused by random bits of comet debris
spread throughout the inner solar system -- the same cloud of
debris that gives rise to the elusive Zodiacal Lights. As our
planet plows through this dusty background, meteoroids hit the
atmosphere and burn up, resulting in a low-level meteor shower
that lasts all year long. Usually sporadics play second
fiddle to short-lived, intense meteor showers like the Leonids,
but this month they take center stage with a modest peak of their
own.

Above: The faint trangular glow in the center of this 1997
picture is sunlight scattered from interplanetary dust particles,
the same particles that cause sporadic meteors. The diffuse "Zodiacal
Lights" are best seen from dark-sky observing sites
before dawn in autumn or just after sunset in the spring.

To understand why sporadic activity is greatest near the beginning
of Fall, simply recall the last time you drove through a swarm
of insects. (Splat! There goes another bug on the windshield...)
Bugs rarely "splat" on the rear window because it's
hard for insects to overtake a fast-moving car from behind. They
accumulate instead on the front glass, in the direction that
the car is moving.

The same holds true for sporadic meteoroids. They usually
meet the Earth in a head-on collision from the direction of our
planet's orbital motion around the Sun, a direction that astronomers
call "the Earth's apex." The region of sky around the
apex is our planet's "front windshield." In late September
the apex, as seen from northern latitudes, lies 70 degrees above
the horizon at dawn -- that's its highest altitude of the year.
With the Earth's "front windshield" so favorably placed,
sporadic meteors are easy to see. Northern observers usually
count twice as many sporadics in September as they do in March,
when the apex has a lower declination.

Above: This sky map shows the direction of Earth's apex
on Sept 22, 2000, the date of the northern autumnal equinox.
The apex is always 90 degrees westward from the Sun along the
ecliptic plane.

The situation is reversed in the southern hemisphere where
the apex is highest in March and lowest in September. This month
happens to be a poor one for sporadic meteor watching south of
the equator.

No matter where you live, north or south, the best time to
spot sporadic meteors is during the hours just before dawn around
early autumn in your hemisphere. Although sporadics tend to come
from the direction of the apex, they can appear in any part of
the sky, headed in any direction. Unlike organized cometary debris
streams that give rise to meteor showers such as the Perseids
or Leonids, sporadic meteoroids are truly random. You never know
what you will get from a sporadic meteor -- you might spy a glorious
fireball, or a flurry of shooting stars, or nothing at all. The
only way to find out is by looking.

If, however, you happen to be looking from inside your car,
we suggest pulling to the side of the freeway. Watching meteors
and chatting on the cell phone at the same time is simply too
dangerous!